The present invention relates to a pretensioner which is installed in a seat belt device for restraining an occupant to a vehicle seat and which rapidly rotates a reel shaft of a seat belt retractor in the belt winding direction. More particularly, the present invention relates to a pretensioner which achieves more stable operation and reduction in power loss of a gas generator.
Vehicles such as automobiles are equipped with seat belt devices. In recent years, the ratio of seat belt devices with pretensioners has been increasing. The function of a pretensioner is applying tension to a seat belt to remove slack of the seat belt by rapidly rotating a spool (reel shaft) on which the seat belt is wound. A typical type of pretensioner has explosive which is detonated in accordance with a signal emitted in the event of collision and drives a mechanism for rotating a spool with gas pressure derived from explosion of the explosive.
The pretensioner may include a pipe having a gas generator fixed to one end. Arranged inside the pipe are a piston and a plurality of balls. At the other end of the pipe, a guide block is fitted. A ring gear having a plurality of external teeth on the outer periphery thereof and a plurality of internal teeth on the inner periphery thereof is provided in the pretensioner. One or more of the external teeth of the ring gear enter into the pipe so that the front-most ball is in contact with one of the external teeth of the ring gear.
Inside of the ring gear, a pinion is positioned to be fitted to the spool gear of the spool. Internal teeth of the ring gear and the external teeth of the pinion are designed to be meshed with each other. When the gas generator is activated to generate gas, the balls are pushed via the piston by the pressure of gas. The ring gear is forced to rotate by the balls and the internal teeth of the ring gear and the external teeth of the pinion mesh together. Thus, the spool rotates via the pinion coupled with the ring gear. In this manner, the seat belt is pretensioned.
As tension exceeding a predetermined value is exerted to the seat belt, the spool is gradually rotated in the belt-unwinding direction with twisting of a torsion bar. Because of this torsional deformation of the torsion bar, impact to be applied to the occupant by the seat belt is absorbed and reduced (i.e. Energy Absorption (EA) Operation is performed).
During the EA operation, the spool rotates in the belt-unwinding direction due to the deformation of the torsion bar, thereby pushing balls to move back toward the gas generator. When gas pressure still exists in the pipe upstream of the balls and downstream of the gas generator, the balls are subjected to the gas pressure. The locations of the balls in the pipe at the start of the EA operation depend on the size and the attitude of the occupant. It is difficult to determine how and to what extent the balls move forward when the EA operation is started.
Therefore, it has been proposed that the pipe include a hole in the side wall located between the the gas generator and the piston to allow gas to discharge. However, due to its location gas may discharge through this hole even at the initial stage of activation of the pretensioner. This may reduce the power of the pretensioner and cause a problem.
Therefore there remains a need to provide a pretensioner which achieves more stable operation and provides for reduced power loss.
According to the present invention a pretensioner which rotates a spool of a seat belt retractor to pretension a seat belt in the event of an emergency is provided. The pretensioner includes a gas generator; a pipe connected to the gas generator and positioned to receive generated gas, and a plurality of balls which are arranged in series in the pipe to be accelerated by the generated gas in a direction away from the gas generator. The ball which is nearest to the gas generator is a piston which is designed to provide gas-tight seal relative to the wall of the pipe. The pretensioner further includes a means for converting the movement of the accelerated balls to a force for rotating the spool. The means includes a clutch for coupling the pretensioner to the spool. The pretensioner further includes a gas release hole formed in the pipe in such a position that before the piston moves the piston is between the hole and the gas generator and when the coupling of the clutch is completed by the movement of the accelerated balls and the piston the hole is located between the gas generator and the piston.
Preferably, the means for converting includes a ring gear having external teeth for receiving at least one of the plurality of balls. The ring gear may include internal teeth for engaging a pinion connected to the spool. The pretensioner may also include pins preventing that prevent the ring gear from engaging the pinion prior to activation of the gas generator.
In another embodiment of the present invention a seat belt pretensioner comprising a plurality of balls positioned in a pipe is provided. The pipe is connected to a gas generator configured so that when generated gas enters the pipe the balls are accelerated toward a clutch mechanism for converting the movement of the balls into rotation of a spool for a seat belt. The pipe includes a hole in a wall positioned so that before the balls move the hole is sealed off from generated gas by the ball closest to the gas generator. When the clutch mechanism begins to effect rotation of the spool the hole is located between the gas generator and the ball closest to the gas generator thereby allowing gas to escape.
According to the aforementioned embodiment, the ball closest to the gas generator may be a piston providing a gas tight seal relative to an interior wall of the pipe.
In another alternative embodiment of the present invention a gas release mechanism for a seat belt pretensioner is provided. The pretensioner includes a gas generator connected to a pipe having a plurality of balls. The gas release mechanism includes a hole in the pipe positioned so that before the balls move due to the generated gas the hole is isolated from the gas generator by one of the balls. The hole is positioned so that when pretensioning of the seat belt begins the hole is located between the gas generator and the ball closest to the gas generator thereby releasing gas out of the pipe.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.